1. Field of the Art
The present invention relates to a process for preparing ethylene copolymers by copolymerizing ethylene and an .alpha.-olefin having 3-20 carbon atoms in the presence of a specific catalyst.
2. Related Art
Catalysts comprising a magnesium halide, a titanium halide and an electron donor have high polymerization activities of an .alpha.-olefin, and thus a number of such catalysts have hitherto been developed, in which a variety of processes such as a slurry method, a vapor (or alternatively gas) phase method, a solution method or a high pressure bulk method is used. Among these catalysts, certain catalysts are used under the polymerization condition at a polymerization temperature of 140.degree. C. or more. Particularly, it is known in the art that ethylene copolymers excellent in transparency, strength, surface tackiness, heat-sealing property and hot tack of film of the copolymers can be prepared by using such a catalyst in the copolymerization of ethylene and an .alpha.-olefin having 5 or more carbon atoms (Japanese Patent Publication No. 52643/1984, and Japanese Patent Laid-Open Publication Nos. 75910/1984 and 88016/1985).
Unfortunately, it is generally impossible, however, to prepare a copolymer with such excellent properties of ethylene and an .alpha.-olefin having 5 or more carbon atoms at a relatively low temperature such as 95.degree. C. or less with the aforementioned catalysts. This is probably because the use of an organoaluminum halide as an activating agent a co-catalyst of a Ziegler catalyst at a high polymerization temperature is critical for the expression of such a specific catalyst property and thus the production of preferable ethylene copolymers. Therefore, it will be of quite a value from the aspect of the saving of resources and energy that such specific ethylene copolymers can be prepared by a slurry method or a vapor phase method in which polymerization proceeds with forming a polymer in the shape of particles at a relatively low temperature.
Moreover, it is also well known from Japanese Patent Laid-Open Publication Nos. 148093/1979, 154488/1979, 62292/1979, 52309/1980 and 56110/1980 or Japanese Patent Publication Nos. 20843/1988, 20844/1988 or the like that a catalyst system comprising a magnesium halide, a titanium halide and a trialkylaluminium has a high activity in the copolymerization of ethylene with propylene or butene at a low temperature and is excellent in copolymerizability (comonomer incorporation), so that it can be preferably applied to a polymerization method in vapor phase or in slurry in which polymerization generally proceeds with forming a copolymer in the shape of particles at a polymerization temperature of 100.degree. C. or less. There was, however, a problem that a copolymer having a density of 0.920 g/cm.sup.3 or less was prepared and fabricated into film particularly in the copolymerization of ethylene with butene with use of such a catalyst system, the copolymer usually exhibited a severe surface tackiness. Thus, additives of inorganic compounds such as silica, zeolite, calcium carbonate or the like have been usually incorporated for the purpose of reducing the tackiness. The additive must be used in a large amount for reducing the surface tackiness to a desired level, and thus serious problem remains in the production cost in such a method.
According to the researches of the present inventors, it has been found that the excellent features of the aforementioned ethylene copolymers are derived from the characteristics of the primary structure of the copolymers, and that these characteristics typically appears in the melting point of the copolymers. In other words, a preferred copolymer prepared in the high temperature range of 95.degree. C. or more as described above has several melting points (endothermic peaks), one of which melting points (endothermic peak) appears at a relatively low temperature range (ca. 90-115.degree. C.), while a copolymer prepared in a low temperature range of 95.degree. C. or less has substantially one melting point (endothermic point) at a relatively high temperature range (ca. 120-126.degree. C.). It is believed from these facts that a homogeneous copolymer having a relatively high density and another homogeneous copolymer having a relatively low density are present in the copolymer prepared at a high temperature range as described above and the latter largely contributes to the improvement of the transparency, strength, heat-sealing property and hot tack.
It has been reported that the aforementioned qualities of copolymers such as the transparency, strength, surface tackiness or heat-sealing property of film of the copolymer relate to the composition distribution of the copolymer and that the qualities are more excellent as the distribution becomes narrower [Y. Sakota, Speciality Plastics Conference 1988, Nov. 28-30, Zuerich, "VLDPE for extrusion coating"- polyethylene and polypropylene resins, markets and applications - Maack Business Services (Switzerland)]. The composition distribution of the copolymer can be determined by the TREF method, which is a conventional technique for the analysis of the composition distribution of a copolymer: Macromolecules, 19, 2722-2726 (1986). It is also possible to determine the composition distribution by analyzing a copolymer according to the DSC method.
Such a copolymer as described above has a plurality of melting point peaks in the determination of the melting points according to the D C method, one of which melting points appears at a relatively low temperature (90-115.degree. C.) and others appear at relatively high temperature (115-126.degree. C.). It has been reported by S. Hosoda in Polymer Journal, 20 (5), 383-397 (1988) that the ratio of the low temperature peak/high temperature peaks conveniently expresses the composition distribution, and that the composition distribution is narrower as the area ratio is larger. The composition distribution can also be expressed by the peak height ratios (if a plurality of peaks are present at the higher temperature side, the highest peak is used for the calculation of the ratio).